Devices for laboratory and on-line moisture or density measurement and identification of agricultural and other products are desirable. Two such devices which are currently available are capacitance-based sensors and microwave sensors, both of which depend on the correlation between dielectric properties and moisture content or thickness. Capacitance sensors are less expensive and easier to work with than microwave sensors, but their accuracy is less than that provided by microwave sensors. Moreover, they are largely restricted to materials with uniform dielectric properties.
The simplest type of capacitor comprises two parallel conducting plates separated by a non-conducting dielectric material. Any dielectric material which is not a vacuum will produce a larger capacitance than if a vacuum were used. Accordingly, a measurement of capacitance can provide information about the dielectric material. Capacitance sensors measure the capacitance for various dielectric materials or samples. When an AC electric field is imposed on a dielectric, it will absorb some of the field""s energy. The loss factor is a measure of the amount of energy absorbed by the dielectric.
The dielectric properties of agricultural products are affected by a number of factors including moisture content, frequency, temperature, and density. Most existing capacitance based sensors measure the dielectric constant and/or loss factor of a sample at a single frequency and use this information to determine material properties such as thickness, density, or moisture content. The present invention relates to a multiple frequency capacitance sensor which measures the dielectric properties of a sample over a wide range of frequencies.
Recent advances in microprocessor technology have made possible many sensor technologies. One such technology is real-time, multiple frequency/multiple parameter dielectric or capacitive sensing which requires significant data storage and analysis. Real-time multiple frequency/multiple parameter dielectric sensing offers tremendous possibilities in material identification, improved precision of composition measurements such as moisture content, and simultaneous identification of multiple material parameters such as moisture and density.
Multiple-frequency sensors for sensing the type of material in a sample are well known in the patented prior art as evidenced by U.S. Pat. No. 5,521,515. As disclosed therein, a voltage-controlled oscillator drives a capaciflector type shielded sensor element. The impedance magnitude is measured over a predetermined frequency range and a digital computer compares the impedance v frequency curve to stored curves of known materials in order to identify the material and as an option to measure the distance to the sample.
Shunt-mode capacitive sensing is also known for various applications but is new to the field of multiple-frequency dielectric properties measurement. Shunt-mode capacitive sensing measures the capacitance between a transmitter and a reference irrespective of any capacitance to a reference potential, rather than between a transmitter and a reference potential. This allows a simple approach to shielding, using a passive shield connected directly to a reference potential rather than an active shield which follows the transmitter""s electrical potential. In addition, it allows more distinct measurements to be made for a given sensor array configuration than a sensor which measures capacitance to a reference potential, as measurements can be made from any sensor element to any other sensor element in an array rather than simply from each sensor element to a reference potential.
Multiple-parameter impedance measurement for real and imaginary portions of complex impedance is also known in the prior art. For example, single-frequency, two-parameter microwave measurements have been used in moisture sensors to allow thickness compensation with known density or density compensation with known thickness. It is known in the field of moisture sensing that two-parameter measurements generally can be used to achieve superior measurement accuracy when compared to single-parameter measurements of the same material.
Capacitive density measurements are also known in the prior art, though such systems have involved measurements at only a single frequency, limiting their effectiveness to materials which have known dielectric properties. Such devices would not be useful with hydroscopic materials such as cereal grains, which have dielectric properties which vary substantially with moisture content.
Accordingly, it is a primary object of the present invention to provide a method and apparatus for determining the bulk density and/or moisture content of a particulate sample using a capacitance sensor. At least one sinusoidal voltage signal is applied to the sensor which is arranged in a quantity of the sample to produce a change in the electrical properties of the sensor as a function of the sample. The change in the electrical properties is measured and then correlated to determine the bulk density and/or moisture content of the sample.
According to another object of the invention, the sensor is a shunt-mode dielectric sensor and the electrical properties include at least one of capacitance and dielectric loss.
The sensor may comprise at least two spaced parallel conductive plates or at least two coplanar conductive plates, while the sinusoidal voltage signal is produced by a digitally synthesized oscillator. Correlation is performed in a microcomputer using algorithms to determine the moisture content and density of the sample.